1. Field of the Invention
This invention relates to gelling agents and to methods of preparing concentrated slurries of polymeric viscosifiers for use in the oil and gas drilling industry. More particularly, the invention relates to a method of treating natural and synthetic water soluble polymer solids with suspending agents so that the polymers can be suspended in a hydrocarbon carrier fluid.
2. Description of the Related Art
Natural and synthetic polymer solids are useful as viscosifiers of aqueous solutions. Viscosified aqueous solutions are used in the oil and gas drilling industry, particularly as frac fluids and as drilling muds.
Frac fluid is pumped into a well to induce or expand cracks in the earthen formation. Causing formation fractures generally will stimulate well production. Water has been used as a frac fluid because it is inexpensive. However, alone, water is not particularly effective at creating sufficient crack width. A more viscous fluid is needed.
Viscosified fluids are also useful as drilling muds. Drilling muds are viscous fluids that maintain pressure against bore holes during drilling and prevent bore holes from caving in. Due to the hydrostatic pressure they impart they also prevent wells from producing excess natural fluids.
Drilling muds and frac fluids are viscosified by adding a viscosifier to an aqueous solution. Consequently, substances have been developed that viscosify water. Common viscosifiers are natural and synthetic water-soluble polymers. These viscosifiers are also referred to as gelling agents. Generally, a gelling agent is a high molecular weight, water-soluble or dispersible polymer. The term is intended to mean polymers which form colloidal solutions or dispersions in water.
Ideally, in an aqueous solution, each gelling agent particle should be separated and wetted. Gelling agents, however, are very difficult to disperse in aqueous liquids. For example, in conventional processes for dissolving gelling agents, dry powdered polymer is fed slowly and carefully into the vortex of a vigorously stirred aqueous solution. Directly adding dry polymer into the aqueous solution often results in undesirable agglomerations of polymer, which are called "fish eyes". Fish eyes are polymer masses wetted on the outside, but with dry, unhydrated material inside.
In the field, addition of a gelling agent to a solution is particularly tedious and problemsome. Often, for example, personnel simply dump the powder into the aqueous solution, start the mixing equipment, and expect the material to disperse or dissolve. Under these circumstances, excessive numbers of fish eyes often result. Mechanical feeders and eductors can lead to better polymer separation and wetting. However, such machinery is inefficient and frequently fails to prevent formation of fish eyes, particularly when used with warm or hot water.
Because of the problems related to mixing and dispersing gelling agent, improved methods have been developed. Sometimes the dry powdered polymer is treated with a surface active material that enhances the penetration of liquid into the gelling agent so that particles separate. Water-soluble inert salts, such as sodium chloride or effervescing salts, may also be used to separate and reduce the agglomeration of gelling agent particles. Certain gelling agents, such as the various cellulose ethers, xanthan gum, and guar gum, have been treated with temporary, removable crosslinking agents to decrease the gelling agent's hydration rate in aqueous solutions. Cross-linking allows gelling agents to disperse into solution before they hydrate.
Many problems associated with suspending gelling agents in an aqueous liquid have been overcome by adding the gelling agent to an aqueous solution as a liquid gel concentrate (hereinafter "LGC") instead of as a solid powder. An LGC, as defined here, is a concentrate of gelling agent suspended in a hydrocarbon carrier fluid. LGCs typically contain: (1) a gelling agent; (2) a hydrocarbon carrier fluid such as a diesel fuel, mineral or vegetable oil; and (3) suspending agents. They sometimes also include a surfactant for enhancing the release of the polymer into an aqueous solution. When the LGC is added to an aqueous solution, the gelling agent is released from the carrier fluid, and disperses into the aqueous solution efficiently and effectively. LGCs are described in Harms et aI, U.S. Pat. No. 4,772,646 (hereinafter the "Harms '646 patent").
Important advantages of using LGCs include savings of equipment, materials, and manpower. These advantages are described in Harms et al, DieseI-Based HPG Concentrate Is Product of Evolution, Petroleum Engineer International pp. 51-54 (April 1988). By themselves, water-soluble gelling agents are not particularly active in a hydrocarbon carrier fluid. Without suspending agents any suspension of the gelling agent is temporary--the solid gelling agent in hydrocarbon liquid ultimately drops through the liquid, leaving a layer of carrier fluid and a layer of sludged solids.
Thus a suspending agent is an important component of an LGC. In the conventional method of preparing an LGC, the suspending agent is admixed with the hydrocarbon fluid separately from the admixing of the gelling agent with the hydrocarbon fluid.
A good suspending agent generally does at least two things. First, it increases, at least to a limited degree, the hydrocarbon's viscosity. However, increased viscosity alone is not enough to suspend solids. So, second, suspending agents also cause the gelling agents to resist agglomeration. The mechanism by which this occurs is poorly understood. It is possible that the suspending agents increase mutual repulsion between polymer solid particles by altering electronic charges on the solid surfaces. It may also be that the suspending agents form films around the individual solid particles that prevent the solids from contacting each other. Regardless of the mechanism involved effective suspending agents cause solids to become more uniformly dispersed in the carrier fluid.
Typical suspending agents include (1) organophilic clays, such as Bentones.RTM. and Claytones.RTM.; (2) hydrophobic silicas, such as Cab-O-Sil.RTM., and fumed silica; and (3) oil-compatible polymer suspending agents, such as is described in Dymond, U.S. Pat. No. 4,670,501 (hereinafter the "Dymond '501 patent"). Combinations of these three kinds of suspending agents can also effectively suspend gelling agents in hydrocarbon.
Of particular interest to the present invention are the suspending agents described in the Harms '646 patent, which are used to prepare LGCs. See also the Dymond '501 patent, which describes preparing oil compatible suspending agents. A commercial suspending agent featured in the '501 patent is GS-5 available from Halliburton Services, Duncan, Okla. Generically, GS-5 is a copolymer comprising primarily a branched chain alkylacrylate and acrylic acid.
Unless indicated otherwise, the term "suspending agent," as used herein, means oil-compatible suspending agents, such as those described in the Harms '646 and Dymond '501 patents.
Until the present invention, certain drawbacks were associated with the use of oil-compatible polymer suspending agents. For example, commercially available compounds generally are prepared via aqueous emulsion polymerization; the emulsion generally has a high water content. The water can react undesirably with gelling agents, causing them to lump, sludge, or alter viscosity performance.
Similarly, the LGC can become contaminated with water in storage by humidity or condensed moisture. Unstable LGCs often result. Bleed-out can occur in a matter of hours to several days after preparation. Other problems also exist. For example, in the conventional field method of preparing an LGC from a gelling agent such as hydroxypropylguar (hereinafter "HPG") and the suspending agent GS-5, the GS-5 is injected into a centrifugal pump to rapidly divide and disperse the GS-5 emulsion material over the hydroxypropylguar solids flowing in diesel. If GS-5 is added without rapid dispersion, an inferior concentrate with agglomerated polymer particles results. In the field, this occurs when personnel pour GS-5 into an open hatch rather than inject it into a centrifugal pump.
Other problems pertain to field preparation of LGCs. Because an LGC is multi-componented, numerous opportunities exist for improper preparation of the LGC--mismeasurement of any component can yield an ineffective LGC. For field use, it is therefore desirable to have all the solid components of the LGC as a single powder, or "one-bag," mixture that can be added directly to the carrier fluid. It is also desirable for the LGC to be storage stable and to require a minimum amount of suspending agent.